1. Field of the Invention
The present invention relates to a piezoelectric resonator which takes advantage of thickness longitudinal vibration. In particular, the present invention relates to a piezoelectric resonator having a structure in which a tabular piezoelectric material with a pair of excitation electrodes disposed on a pair of principal surfaces stands upright on a substrate, as well as a method for manufacturing the same, a piezoelectric filter including the piezoelectric resonator, and a duplexer.
2. Description of the Related Art
In recent years, as the frequencies of communications equipment have increased, various piezoelectric resonators and piezoelectric filters taking advantage of thin piezoelectric materials and piezoelectric thin films have been proposed. For example, FIG. 1 is a perspective view partially showing a piezoelectric resonator disclosed in Japanese Unexamined Patent Application Publication No. 2001-44794.
In a piezoelectric resonator 101, a piezoelectric plate 103 made of PZT stands upright on a sapphire substrate 102. Excitation electrodes 104 and 105 are disposed on opposing surfaces of the piezoelectric plate 103. The piezoelectric plate 103 is polarized in the direction in which the excitation electrodes 104 and 105 are opposite to each other, that is, in the thickness direction. Therefore, resonance characteristics taking advantage of thickness longitudinal vibration can be attained by applying an alternating-current electric field from the excitation electrodes 104 and 105. As for the above-described piezoelectric resonator 101, the frequency can be increased by decreasing the thickness of the piezoelectric plate 103.
In the production of the piezoelectric resonator 101, a sapphire substrate 102 is prepared, and a film of PZT piezoelectric ceramic is formed on the top surface 102a of the sapphire substrate 102 to reach a height equal to the height of the top surface 103a of the piezoelectric plate 103. Subsequently, the resulting piezoelectric film is processed by etching or the like to form the piezoelectric plate 103 having a thickness of about 2 μm or less. After the piezoelectric plate 103 is formed, the excitation electrodes 104 and 105 are formed on both surfaces of the piezoelectric plate 103. In this manner, the piezoelectric plate 103 having a reduced thickness can be disposed upright on the sapphire substrate 102. It is believed that a piezoelectric resonator usable in a GHz band can be provided since the piezoelectric plate 103 has the reduced thickness.
However, the thin piezoelectric plate 103 in the piezoelectric resonator 101 described in Japanese Unexamined Patent Application Publication No. 2001-44794, is composed of the PZT piezoelectric ceramic. Since the PZT piezoelectric ceramic contains three elements, Pb, Ti and Zr, as metal atoms, there is a problem in that the composition is hard to stabilize. That is, variation in performance tends to occur since the composition of the piezoelectric plate 103 is not stabilized.
In addition, the piezoelectric plate 103 is made of the piezoelectric ceramic and, therefore, as described above, a treatment step of polarizing the piezoelectric plate 103 in the thickness direction must be performed while the piezoelectric plate 103 stands upright. Consequently, the production process is complicated.